1. Field of the Invention
The present invention relates generally to a transmit diversity system, and in particular, to a system in which a UTRAN supporting an N-antenna transmission diversity scheme is compatible with a UE for providing an M-antenna transmit diversity system.
2. Description of the Related Art
As mobile telecommunication technology progresses rapidly resulting in an increase of the amount of service data that can be accommodated, a 3rd generation mobile telecommunication system has been developed for high-speed data transmission. The 3rd generation mobile telecommunication system has been separately standardized into an asynchronous W-CDMA (Wideband Code Division Multiple Access) or UMTS (Universal Mobile Telecommunication System) system in Europe and a synchronous CDMA-2000 system in North America. Such a mobile telecommunication system is generally configured such that a plurality of UEs (User Equipments) communicate with one another through one UTRAN (UMTS Terrestrial Radio Access Network). In the mobile telecommunication system, a received signal is subject to phase distortion due to fading which occurs in a radio channel during high-speed data transmission. The fading causes attenuation in amplitude of the received signal from several dB to several tens of dB. If not properly compensated for during data demodulation, the phase of the received signal, distorted by the fading, becomes a cause of an information error on the data transmitted from a transmission side, decreasing the quality of service (QoS) of the mobile telecommunication system. In order to transmit the high-speed data without deterioration of the service quality, the mobile telecommunication system must resolve the fading problem and to do so, employs various diversity techniques.
In general, the CDMA system adopts a Rake receiver, which performs diversity receiving using delay spread of a channel. Although the Rake receiver performs receive diversity for receiving a multi-path signal, a diversity technique using the above-stated delay spread does not operate desirably when the delay spread is less than a preset value. In addition, a time diversity technique using interleaving and coding is used in a Doppler spread channel. However, it is difficult to use the time diversity technique in a low-speed Doppler spread channel.
Therefore, in order to resolve the fading problem, a space diversity technique is used in a low-delay spread channel such as an indoor channel and in a low-speed Doppler spread channel such as a pedestrian channel. The space diversity technique uses two or more transmit/receive antennas. That is, when a signal transmitted through one antenna is attenuated due to the fading, the space diversity technique receives a signal transmitted through another antenna. The space diversity can be classified into a receive diversity technique using a receive antenna and a transmit diversity technique using a transmit antenna. Although the receive diversity technique is applied to the UE, it is difficult to mount a plurality of antennas on the UE, taking into account the size and cost of the UE. Therefore, it is recommended to use the transmit diversity technique in which a plurality of antennas are mounted on the UTRAN.
The transmit diversity technique relates to an algorithm for obtaining a diversity gain by receiving a downlink signal, and is divided into an open loop mode and a closed loop mode. In the open loop mode, the UTRAN transmits a data signal through the diversity antennas after encoding, and the UE then receives the signal transmitted from the UTRAN and decodes the received signal, thereby obtaining a diversity gain. In the closed loop mode, (1) the UE predicts channel environments to which the signals transmitted through the respective transmission antennas of the UTRAN will be subjected, (2) the UE calculates weights that are proper to maximize power of the received signals, for the antennas of the UTRAN depending on the predicted values and transmits information on the calculated weights to the UTRAN through an uplink and (3) the UTRAN then controls weights of the respective antennas based on the weight information transmitted from the UE. For channel measurement of the UE, the UTRAN transmits pilot signals assigned to the respective antennas, and the UE measures the channels through the pilot signals and determines optimal weights using this channel information.
U.S. Pat. No. 5,634,199, entitled “Method of Subspace Beamforming Using Adaptive Transmitting Antennas with Feedback” and U.S. Pat. No. 5,471,647, entitled “Method for Minimizing Cross-talk in Adaptive Antennas”, disclose a method for using the transmit diversity technique in a feedback mode. U.S. Pat. No. 5,634,199 discloses a channel measurement and feedback method using a perturbation algorithm and a gain matrix. However, this method, being a blind algorithm, is not used often since it has a low conversion speed for channel measurement and has a difficulty in finding correct weights.
Meanwhile, UMTS, i.e., W-CDMA (3GPP (3rd Generation Partnership Project)) Release 99 recommends a method for quantizing weights of two antennas and feeding back the quantized weights. This method refers to a case where there exists only the UE supporting two-antenna transmit diversity techniques. That is, the W-CDMA Release 99 does not mention a transmission method for the case where the UTRAN has four transmit antennas, nor a UTRAN signal transmission method and a signal reception method of the UE, considering the case where there coexists one UE employing two-antenna transmit diversity techniques and another UE employing four-antenna transmit diversity techniques. If the transmit antennas are expanded to four in number using a method for expanding the conventional method for transmitting a signal through a single antenna to a method for transmitting a signal through two transmit antennas, the UE employing the two-antenna transmit diversity techniques will not operate normally. If one method for transmitting a signal with two antennas and another method for transmitting a signal with four antennas are both used to resolve the above-stated problem, a new problem of power imbalance between the antennas will arise.
A method for transmitting different pilot signals through a plurality of antennas includes a time division multiplexing (TDM) system, a frequency division multiplexing (FDM) system and a code division multiplexing (CDM) system. To transmit the different pilot signals through the antennas, the W-CDMA system can use scrambling codes, channelization codes or orthogonal pilot symbol patterns.
In general, the system using two transmit antennas can obtain a considerably high diversity gain and a signal-to-noise ratio (SNR) of up to 3 dB, compared with existing systems using a single antenna. In addition, when the transmit diversity technique uses more than two antennas, the diversity gain increases to a level higher than the diversity gain obtainable by the two-antenna transmit diversity technique, and the SNR also increases in proportion to the number of antennas. Here, the increased diversity gain is relatively lower than the diversity gain obtained by the two-antenna diversity technique, but a diversity degree increases so that an increase in the signal-to-noise ratio (Eb/No) causes an increase in the diversity gain.
The W-CDMA Release 99 for the UMTS system currently discloses a transmit diversity technique using only two antennas. However, the W-CDMA Release 99 considers a necessity of a transmit diversity technology using more than two antennas. That is, it must consider a mobile communication system in which there coexists an existing UE receiving signals transmitted from two transmit antennas and a UE receiving signals transmitted from more than two transmit antennas. In this case, a transceiver is required which is structured such that the UE using two-antenna transmit diversity technique and the UE using the more-than-two-antenna transmit diversity technique can normally receive signals from the UTRAN. That is, a transmitting/receiving method and apparatus must be considered, which operates normally even when the UE designed to accommodate a UTRAN system employing the two-antenna transmit diversity technique is located in a service area of a UTRAN system employing the more-than-two-antenna transmit diversity technique. On the other hand, a transmitting/receiving method and apparatus must be also considered, which operates normally even when the UE designed to accommodate the UTRAN system employing the more-than-two-antenna transmit diversity technique is located in a service area of the UTRAN system employing the two-antenna transmit diversity technique. In addition, it is necessary to provide compatibility with the UE designed to accommodate the UTRAN system employing the transmit diversity technique using more than two antennas, without modification of the UE designed to accommodate the UTRAN system employing the two-antenna transmit diversity technique.
Compatibility is especially required in the common pilot channel, and a common channel for transmitting common data. This is because although a dedicated channel may transmit signals in a proper diversity method depending upon the characteristic and version of the UE, a common pilot channel (CPICH) and a common data channel, which are common channels, must be constructed so as to support both a lower-version UE operating in the UTRAN system employing the existing two-antenna transmit diversity technique and an upper-version UE operating in the UTRAN system employing the more-than-two-antenna transmit diversity technique. That is, the common channels must have higher reliability for the signals transmitted by the system, compared with the dedicated channel, so that the common channels transmit the signals at higher power compared with the dedicated channel. Therefore, it is possible to perform communication at lower transmission power by obtaining a transmit diversity gain from the common channels, thereby increasing an overall system capacity, i.e., the number of subscribers.
The transmit antenna system is a system which transmits signals with a plurality of antennas. A transmission RF system including an antenna power amplifier, e.g., a low noise amplifier (LNA) is advantageous in terms of cost and efficiency, when power of the signals transmitted through a plurality of antennas is uniformly distributed. If the transmission power is non-uniformly distributed to a specific antenna, it is difficult to design the antenna and the cost increases undesirably. It is difficult to provide the compatibility with the method and apparatus employing the two-antenna transmit diversity technique, if the transmission/reception system is not efficiently designed.